This invention is directed to the field of radiant electromagnetic energy collectors, particularly those for collecting solar energy.
A large amount of interest has evolved in recent years over the use of solar energy as an alternative energy source. Among the problems encountered in the design of radiant energy collectors has been the need for a reflecting and concentrating system which can efficiently collect radiant energy from a wide range of directions thus avoiding the necessity for complicated tracking gear to readjust the facing of the energy collector as the sun traverses the sky during the day and as the sun's height in the sky changes with the seasons. Such a design is disclosed in U.S. Pat. No. 4,002,499 to R. Winston. The Winston design has an additional advantage in that it is a non-imaging system which operates efficiently to concentrate both specular and diffuse radiation and thus provides an advantage in hazy weather when much of the solar radiation is scattered and beam radiation is at a minimum.
Another important factor in collector design concerns the absorber which must absorb as much incident radiation as possible while keeping heat loss due to reradiation, convection and conduction to a minimum. A very efficient absorber design for achieving these goals has a helically coiled pipe coated with a radiation absorbing surface positioned in an evacuated glass tube. A heat exchange fluid in the pipe absorbs incident radiation while the coating keeps reradiation to a minimum and the evacuated space prevents heat loss due to convection and conduction. Such a heat absorber has not been proposed for use with a non-imaging concentrator, however.
An area in the design of elongated trough-like radiant energy collectors which has received little attention involves the end portions of the energy concentrator. The ends are typically simply cut off and closed off with a flat plate which serves no concentrating function. This is a disadvantage for a solar collector trough oriented with its longitudinal axis on an East-West axis since the sun is usually not projecting its rays at a perpendicular angle with respect to the longitudinal axis. Thus it is most often the situation that an energy absorber equal in longitudinal length to the concentrator will have concentrated rays directed onto only one of its end portions while the incident rays at the opposite end of the trough will be reflected and concentrated towards an area on the absorber longitudinally displaced from the absorber end. Thus the entire length of the absorber will not be utilized. If the end plates are opaque they present an added disadvantage in that at different times of the day one or the other of them will cast a shadow on the absorber.